It seems to me there's lots of evidence they fear this and little evidence it's justified.

I think modern systems are viewed as constrained platforms and the fear at a structural level is people doing interesting unconstrained things with then. For apple that's an opportunity loss: if ableton on Linux is better they can't monetise ableton on OSX as successfully, is about as bad as it gets.

Nobody sues dell or lenovo because a PC runs Linux and the bits rot.